Optical scanners conventionally work by illuminating an object to be scanned with a light source and recording the reflected light from the object with an image sensor. Examples of image sensors are charge coupled devices (CCD) or contact image sensors (CIS). The object may be separated from the image sensor and the light source by a glass plate. The image sensor is typically arranged on a movable fixture, moving past a medium to be scanned, or the medium is pulled past the image sensor by motor-operated rolls. Thereby a complete scanned image of the physical medium can be generated. The scanned image is influenced by the intensity and spectrum of the light source and the sensitivity of the image sensor. As the intensity and/or spectrum of the light generated by the light source and/or the sensitivity of the image sensor changes over time, images with faulty colour balances and/or intensity values may result.
WO09138255A discloses a device for optical scanning, comprising: a plurality of image sensor elements, where the image sensor elements are arranged to detect light incident on the image sensor elements, emitted from a light source and reflected from an object subject to optical scanning; wherein the amount of light received at least one of the image sensor elements is to a larger extent light transmitted from the light source; and to a smaller extent light reflected from the object. Thereby some variations in the light generated by the light source may be accounted for. Further studies have, however, discovered the need for an improved method of compensating for light variations.
Thus, it remains a problem to provide a more effective method for compensating for light variations of light sources in optical scanners.